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but if you keep their number down and intercontinental travel by boat is small in volume, it's better.
Whatever number of people you need to move, air liners are still at least a whole order of magnitude less poluting than ocean liners.
Cutting travel volume by banning the most efficient method is ridiculous.
gbj_tester-
It may seem that way, but unfortunately the only way of reducing unsustainable transport is to make it less convenient. Much of that involves adding time, hence water-based is best. Needless to say, I'm not in favour of ocean liners, either. Small to medium ships would be best.
Obviously, it's completely counter-intuitive in the age of hypermobility, but all motorised transport is unsustainable, whether electric or other kinds of fuel-based. The only way is reduction and abandoning our science fiction-inculcated ideas about machine travel on demand.
Oliver Schick
The main reason for preferring slow travel to fast is that the volume goes right down. It's the constant, everyday grind of planes blasting off, because everybody thinks they ought to be able to go to the other side of the world at the drop of a hat, that's the main problem. In themselves, as tester says, operating large ships is very environmentally damaging, e.g. in the (pre-COVID, although I expect it'll go back to how it was before pretty quickly once we feel we can live with the virus) burgeoning cruise ship industry, but if you keep their number down and intercontinental travel by boat is small in volume, it's better. Too many ifs, and equally obviously, people will resent any such curtailing of their modern 'freedom'.